Note also that this is not the first XFS disk format change; XFS can handle such changes gracefully. But not all changes are compatible in both directions; trying to maintain forward & backwards compatibility across all features is a trail of woe.
However the statement "no forward or backward compatibility" sounds a bit extreme; old filesystems can still be mounted & run by the newer code, the new features just won't be there, and there won't be a facility to "upgrade" an older filesystem.
Newer filesystems with newer features won't, however, be mountable by older kernels. Again, not the first time XFS has gone down this path, no forks required.